Numerous applications in engineering and physics require the use of filters to selectively pass and reject various components of an incoming signal. For example, electromagnetic signals such as electrical and optical signals are filtered using filters implemented by electronic circuitry and optical lenses, respectively. Similarly, acoustic signals are filtered using mechanical means. Finally, charge-carrying particles, such as electrons, are filtered by electronic filters.
Filters in each of these regimes invariably suffer from one main problem: the passband of the filter--and, in particular, its center frequency and width--are difficult to control precisely. This is especially true in the case of filters that are intended to be extremely selective, i e., those having extremely narrow passbands. Also, the response time of such filters while extremely fast, is limited by the transit time of the filter medium. A need exists, therefore, for a high-speed filter which has a very narrow passband.